1. Field of the Invention
The present invention relates to integrated circuits and, in particular, to amplifier integrated circuits with frequency compensation capability.
2. Description of the Related Art
Amplifier integrated circuits (ICs) constitute one of the basic types of analog integrated circuits. Amplifier ICs are essentially configured to receive an input voltage signal (Vin), or input current signal, and, in response, provide a larger output voltage signal (Vout), or larger output current signal. Such amplifier ICs can be used, for example, as an audio power amplifier or to drive a cathode ray tube (CRT).
FIG. 1 is a simplified electrical schematic diagram of a conventional cascode amplifier IC 10. Conventional cascode amplifier IC 10 includes an input bias terminal 12, a power supply input terminal 14, an input signal terminal 16 and an output signal terminal 18. In conventional cascode amplifier IC 10, bipolar transistor X5 (configured as a PNP emitter-follower) functions as an input buffer stage circuit. Such an input buffer stage circuit minimizes the current loading of any pre-amplifier devices (e.g., a video pre-amplifier, not shown). Resistor R9 is configured to turn on bipolar transistor X5 when there is no input voltage signal (Vin). Resistor R6 functions as a pull-up resistor for bipolar transistor X5 and also limits the current flow therethrough.
Bipolar transistors X1 and X2 are arranged in a cascode configuration to provide a gain stage circuit. For example, bipolar transistor X1 can be a low voltage and very fast bipolar transistor, while bipolar transistor X2 can be a higher voltage bipolar transistor than X1. The cascode configuration of bipolar transistors X1, X2, in this situation, provides the equivalent of a very fast and high voltage bipolar transistor. Resistors R2, R3 set the current flow through bipolar transistors X1, X2, while the resistance ratio of resistors R2 and R3 sets the gain (i.e., amplification) of conventional high frequency cascode amplifier IC 10. Resistor Rb limits the current through bipolar transistor X2. Bipolar transistors X3, X4 are configured to function as an output buffer stage circuit. Bipolar transistor X6, along with resistors R7, R8, are configured as a bias stage circuit and set the bias current through bipolar transistors X3, X4 when there is no change in the level of the input voltage signal Vin.
For a conventional cascode amplifier IC to successfully operate in the high frequency (i.e., high speed) regime, it should possess a frequency compensation capability that improves the high frequency response of the conventional cascode amplifier IC.
FIG. 2 is a schematic simulation diagram of conventional high frequency cascode amplifier IC 20 with frequency compensation capability. The conventional high frequency cascode amplifier IC 20 is configured to assert an amplified output voltage signal Vout (at output signal terminal 22) in response to input voltage signal Vin (received at input signal terminal 24), when biased by bias voltage Vb (received at input bias terminal 26) and provided with power supply voltage Vcc (received at power supply input terminal 28).
Conventional high frequency cascode amplifier IC 20 includes an input buffer stage circuit 30 that includes bipolar transistor X87, a gain stage circuit 32 that includes bipolar transistors X91, X85, an output buffer stage circuit 34 that includes bipolar transistors X81, X82, X83 and X89, and a bias stage circuit 36 that includes bipolar transistor X84. Conventional high frequency cascode amplifier IC 20 also includes a resistance-capacitance (RC) series circuit 40. RC series circuit 40 is configured to provide for frequency compensation (also referred to as frequency xe2x80x9cpeakingxe2x80x9d) during operation of the conventional high frequency cascode amplifier IC 20. The RC series circuit 40 includes resistors R112, R102 that are electrically connected in series with two metal-polysilicon peaking capacitors C63, C102. As depicted in FIG. 2, RC series circuit 40 provides for frequency compensation by feeding the amplified output signal (Vout) back to an emitter of bipolar transistor X91 of the gain stage circuit 32 of the conventional high frequency cascode amplifier IC 20 through the RC series circuit 40.
One skilled in the art will recognize that FIGS. 1 and 2 are representative of a variety of well known conventional cascode amplifier IC configurations. Further descriptions of cascode amplifier ICs are included in U.S. Pat. No. 5,977,610 to Hon Kin Chiu, xe2x80x9cAnalysis and Design of Analog Integrated Circuits, Third Editionxe2x80x9d by P. R. Gray and R. G. Meyer, pp. 225-226, 464-466 and 511-513 (John Wiley and Sons, 1993), and co-pending application Ser. No. 09/615,527, Hon Kin Chiu, xe2x80x9cCascode Amplifier Integrated Circuit With Reduced Miller Capacitance at an Output Buffer Stage During a Transient Fall Responsexe2x80x9d), each of which is hereby fully incorporated by reference.
A drawback of conventional cascode amplifier ICs with frequency compensation capability is a wide variation in transient rise and fall times. Such a wide variation is due to inherent tolerances in the speed of the bipolar transistors in the circuits, as well as in the capacitance of the metal-polysilicon peaking capacitors. These inherent tolerances are caused by variations in the manufacturing process. Consequently, a given conventional cascode amplifier IC may include either xe2x80x9cfastxe2x80x9d or xe2x80x9cslowxe2x80x9d bipolar transistors and metal-polysilicon peaking capacitors with either a minimum or a maximum capacitance.
There is, moreover, no relationship or tracking between the speed of the bipolar transistors and the capacitance of the metal-polysilicon peaking capacitors. If, for example, the bipolar transistors are xe2x80x9cfastxe2x80x9d and the metal-polysilicon peaking capacitors are at their typical value, the conventional cascode amplifier IC will tend to exhibit a transient overshoot. If, however, the bipolar transistors are xe2x80x9cslowxe2x80x9d and the metal-polysilicon peaking capacitors are at their minimum capacitance, the conventional cascode amplifier IC will exhibit long transient rise and fall times. The overall transient rise and fall time variation for conventional cascode amplifier ICs is, therefore, undesirably large.
FIGS. 3-6 depict the transient rise and fall responses for the conventional high frequency cascode amplifier IC of FIG. 2. FIG. 3 depicts the transient rise response for the circumstances of xe2x80x9cfastxe2x80x9d transistors (curve 3A) and xe2x80x9cslowxe2x80x9d transistors (curve 3B) and metal-polysilicon peaking capacitors with typical capacitance values. For the fast transistors, the transient rise time is approximately 3.40 nano-seconds, while for the slow transistors it is approximately 3.86 nano-seconds.
FIG. 4 depicts the transient fall response for the circumstances of xe2x80x9cfastxe2x80x9d transistors (curve 4A) and xe2x80x9cslowxe2x80x9d transistors (curve 4B) and metal-polysilicon peaking capacitors with typical capacitance values. For the fast transistors, the transient fall time is approximately 3.69 nano-seconds, while for the slow transistors it is approximately 4.23 nano-seconds.
FIG. 5 depicts the transient rise response for the circumstances of xe2x80x9cfastxe2x80x9d transistors with metal-polysilicon peaking capacitors at their maximum value (curve 5A) and xe2x80x9cslowxe2x80x9d transistors with metal-polysilicon peaking capacitors at their minimum capacitance value (curve 5B). For curve 5A, the transient rise time is approximately 3.29 nano-seconds, while for curve 5B it is approximately 4.03 nano-seconds.
FIG. 6 depicts the transient fall response for the circumstances of xe2x80x9cfastxe2x80x9d transistors with metal-polysilicon peaking capacitors at their maximum value (curve 6A) and xe2x80x9cslowxe2x80x9d transistors with metal-polysilicon peaking capacitors at their minimum capacitance value (curve 6B). For curve 6A, the transient fall time is approximately 3.53 nano-seconds, while for curve 6B it is approximately 4.42 nano-seconds.
Based on FIGS. 3-6, the overall variation in transient rise and fall time for the conventional high frequency cascode amplifier IC of FIG. 2 is 1.13 nano-seconds (i.e., 4.42 nano-seconds minus 3.29 nano-seconds). This relatively large overall variation in transient rise and fall times is undesirable.
Furthermore, the use of metal-polysilicon peaking capacitors in conventional cascode amplifier ICs with frequency compensation capability results in a relatively low RC series circuit breakdown voltage and an IC with a relatively large size.
Still needed in the field, therefore, is a cascode amplifier IC with frequency compensation capability that provides for a tight overall variation in transient rise and fall times. In addition, the cascode amplifier IC with frequency compensation capability should be relatively small in size and possess relatively high RC series circuit breakdown voltages.
The present invention provides a cascode amplifier integrated circuit with frequency compensation capability that possesses a tight overall variation in transient rise and fall times. In addition, cascode amplifier integrated circuits with frequency compensation capability according to the present invention are relatively small in size and have relatively high RC series circuit breakdown voltages.
A cascode amplifier integrated circuit (IC) with frequency compensation capability according to the present invention includes an input bias terminal configured to receive a bias voltage Vb, (e.g., a 12 volt bias voltage signal), a power supply input terminal configured to receive a power supply voltage Vcc, (e.g., an 80 volt power supply voltage signal), an input signal terminal configured to receive an input voltage signal Vin and an output signal terminal.
The cascode amplifier IC with frequency compensation capability according to the present invention also includes a gain stage circuit and an output buffer stage circuit. The gain stage circuit is configured to amplify the input voltage signal received at the input signal terminal and to thereby produce an amplified voltage signal. The output buffer stage circuit is configured to receive the amplified voltage signal from the gain stage circuit, increase the current thereof and transmit the resultant amplified voltage signal with increased current to the output signal terminal as an amplified output voltage signal.
Furthermore, the cascode amplifier IC with frequency compensation capability according to the present invention also includes a resistance-capacitance (RC) series circuit configured to provide frequency compensation during its operation. This RC series circuit has a peaking bipolar transistor (e.g., an NPN peaking bipolar transistor) configured to provide a bipolar junction peaking capacitance between the output signal terminal and the gain stage circuit. The bipolar junction peaking capacitance can be provided, for example, as the reverse biased base-collector junction capacitance (Cbc) of an NPN peaking bipolar transistor.
Since the RC series circuit of cascode amplifier ICs with frequency compensation capability according to the present invention employs a single bipolar junction peaking capacitance (instead of a plurality of conventional metal-polysilicon peaking capacitors), any manufacturing induced variation of the RC series circuit capacitance tracks with any manufacturing induced variation in the bipolar transistors of the various circuits (e.g., the gain stage circuit and the output buffer stage circuit). The result is a cascode amplifier IC with frequency compensation capability that possesses a relatively tight variation of transient rise and fall times. In addition, since a bipolar transistor is smaller in size than a combination of any number of conventional metal-polysilicon peaking capacitors, the size of cascode amplifier ICs with frequency compensation capability according to the present invention is relatively small. Furthermore, the collector-base breakdown voltage (BVcb) of the peaking bipolar transistor is higher than the breakdown voltage of conventional metal-polysilicon peaking capacitors.